Plasma torches are commonly used for cutting, welding and spray bonding of workpieces and are operated by directing a plasma consisting of ionized gas particles toward a workpiece. In the operation of a typical plasma torch, a gas to be ionized is supplied to the front end of the plasma torch and channeled between a pair of electrodes before exiting through an orifice in the torch nozzle. One electrode, which is at a relatively negative potential, is usually referred to as the "cathode" or simply as the "electrode". The torch nozzle, which is adjacent to the end of the "electrode" at the front end of the torch or the workpiece, constitutes the relatively positive potential electrode or "anode".
When a sufficiently high voltage is applied, an arc is caused to jump the gap between the electrode and the torch nozzle, thereby heating gas passing around the electrode and between the electrode and nozzle and causing it to ionize. A high frequency voltage between the electrode and the nozzle starts the plasma arc. The ionized gas flows out of the torch and appears as an arc that extends externally from the outlet in the torch nozzle. This is the pilot arc. When this pilot arc is brought near the workpiece, the arc transfers to the workpiece which then serves as the anode. This operation is initiated by the torch head being moved close to the workpiece so the arc jumps or transfers between the electrode and the workpiece.
During the operation of a conventional plasma torch, the torch becomes very hot, especially near the plasma outlet. Therefore, sufficient cooling of the torch is provided during normal operation to prevent structural elements of the torch, such as the electrode and/or the nozzle, from either melting or deteriorating too rapidly.
Examples of cooling plasma arc torches by the use of gas are disclosed in U.S. Pat. Nos. 4,024,373 and 4,558,201. Cooling with gas alone can be adequate to prevent melting or extremely rapid deterioration of the torch structural components. Further, with gas cooling of the torch components, the torch can be portable since it does not require the bulky liquid coolant reservoirs, radiators or heat exchangers and/or complicated piping associated with the use of a recirculating liquid coolant. Still, for safety and economic reasons, improvements in cooling which exceed that provided by gas alone for reducing the speed of deterioration of the torch components and the lowering of the torch operational temperature is always an important factor in plasma arc torch design. Consequently, plasma arc torches have also been cooled with liquid coolants by conventional recirculating systems disclosed in U.S. Pat. Nos. 2,906,857; 3,450,926 and 3,597,649. Cooling with a liquid coolant provides adequate cooling to prevent the torch from overheating and from deteriorating too rapidly. However, water cooling usually requires relatively complicated flush supply and conduit recirculation systems which are more expensive to manufacture than gas cooling systems and often require repair due to the high operating temperatures of the torches and the rough handling during normal usage. Besides the extra expense caused by equipment failure and the resulting down time and lost production costs often associated with water cooled torches, the requirement for a relatively large, bulky coolant supply tank and a relatively fragile heat exchanger prevents torches with this type of cooling from being easily portable.